The tissue culture of plants is utilized for numerous purposes such as the large-scale proliferation of virus-free strains and elite plants and production of new plant varieties. In conventional tissue culture, a mainstream method has been one using the so-called agar medium in which a prescribed medium solution having a fixed composition is formed by agar as its medium.
FIG. 11 diagrammatically shows tissue culture using the agar medium containing nutrients. That is, in such tissue culture, a plant tissue (see FIG. 11(a)) removed by being cut from a plant is first placed in the agar medium in a container and is proliferated therein (see FIG. 11(b)). The germinated tissue is further divided by being cut (see FIG. 11(c)), and is then transplanted in a rooting agar medium in another container and undergoes rooting therein (see FIG. 11(d)). The plant which has thus rooted is removed from the contained, and the agar attached to its root is washed away (see FIG. 11(e)). Subsequently, the plant is moved into an acclimatizer (see FIG. 11(f)). In this acclimatizer, re. rooting is promoted in a highly humid environment with light shielded. Finally, a seedling thus grown is removed from the acclimatizer and is acclimatized to the external environment (see FIG. 11(g)).
Thus, in the conventional tissue culture, in each stage of proliferation, germination, and rooting, separate agar media containing most suitable components are used. Accordingly, it is true that suitable nutrition corresponding to its growth stage is constantly supplied to the plant, and that favorable results are attained. On the other hand, for that purpose, the aforementioned transplanting operations are required in each stage of culture, and in cases where the nutrition in the agar are lacking, the plant needs to be transplanted in new agar, thereby involving substantial manpower and cost. Incidentally, such a transplanting operation is conducted in an aseptic state.
In addition, in the above described conventional plant culture, at the time when the young seedling which has rooted is subjected to acclimatization to the outside, it is necessary to transplant it to an implantation medium such as vermiculite or the like. At that juncture, since sugar is contained in the aforementioned agar medium, the portion of its root is usually washed with water, as described above, for the purpose of preventing the occurrence of mold. On the other hand, however, there arise problems in that the root is liable to become damaged and that its growth is delayed. It goes without saying that the trouble and cost required for washing and transplantation cannot be ignored.
Furthermore, in the above-described conventional method, the root which has developed in the medium is liable to lapse into a state of oxygen shortage, and therefore there are many cases where the plant fails to elongate sufficiently and become teratomatic. Moreover, with this conventional method, the replacement, replenishment, and discharge of the medium during culture are impossible, and therefore it is impossible to constantly optimize the gas and components of the medium.
The present invention has been devised in the light of the above-described circumstances in tissue culture, and the object of the present invention is to provide a liquid culture apparatus which is capable of constantly maintaining the above-described plant tissue (hereinafter referred to as the "plant body" as a generic term for the tissue as well as an adventitious embryo of a shoot primordium into which the tissue has grown) in an optimum environment without requiring any operations such as transplantation in the course of culture including implantation, germination, and rooting, as well as acclimatization to the outside, and which in the end is capable of attaining high-speed, large-scale production at low cost on a stable basis.